Generality
The impedance test is a survey that evaluates the state of health of the external and middle ear .
This test is useful above all to verify the functioning of the structures that make up the sound amplification system (Eustachian tube, tympanic membrane and three ossicles of hearing). In other words, the impedance test of the ear allows us to understand if there are lesions or disorders that can cause abnormal auditory perception . The investigation is ambulatory, lasts a few minutes and does not cause pain.
Impedance measurement is conducted by inserting a probe into the patient's ear. The device emits a sound wave of variable intensity, able to set in motion the eardrum and the chain of ossicles (hammer, anvil and stirrup) attached to it. The results thus obtained are processed and transferred into a graph, whose interpretation is the responsibility of the physician.
What's this
The impedance test is an investigation aimed at the diagnosis of lesions or disorders of the ear that determine a dysfunction of the structures assigned to the amplification of the sound.
Impedance test: synonyms
The impedance test of the ear is also called impedenzometry or tympanometry .
The test consists of two parts:
- Timpanogramma : it measures the impedance from the middle ear, that is the resistance opposed by the tympanic membrane and the chain of the three ossicles of the hearing to the passage of the sound wave. The tympanogram therefore provides information on the degree of elasticity and movement of the eardrum-ossicular system in response to a sound stimulus . At the same time, this part of the impedance test allows us to verify the patency of the Eustachian tube .
- Stapedial reflexogram : it evaluates the presence or not of the reflex of the stapedial muscle, located in the tympanic chest, establishing whether the contraction occurs correctly when the ear perceives sounds of high intensity .
Ear anatomy (in short)
To better understand the purpose of the impedance test, it is necessary to remember some notions related to the structure of the auditory organ.
The ear can be anatomically divided into three parts:
- EXTERNAL EAR: formed by the auricle, made up of skin and cartilage, and of the external auditory canal. Thanks to its structure, the external ear conveys the sound waves towards the membrane of the eardrum, which provides the useful surface for sound collection and vibrates in response to the acoustic stimulus.
- MIDDLE EAR : it is a small cavity placed between the eardrum and the inner ear, to which it transmits the mechanical vibratory energy of the sound through a system of three auditory ossicles (in sequence: hammer, anvil and stirrup). These elements transfer the sound waves in modified form to the oval window, an opening in the bony wall of the middle ear cavity. At this level, there is also the Eustachian tube (or auditory tube), a conduit that connects the auditory organ with the nasopharynx (portion of the throat, behind the nose). The functions it performs are different: it ensures the exchange of the air trapped in the middle ear, balances the external pressure with the internal one (in order to avoid problems of a pressor nature on the tympanic membrane) and favors the drainage of the mucus.
- INTERNAL EAR : it consists of a complex series of structures (vestibular apparatus and cochlea) contained deep in the bones of the skull. The function of the cochlea is acoustic and consists in transforming the sound waves transmitted by the middle ear into electrical impulses which, through the acoustic nerve, are sent to the brain. The vestibular apparatus, on the other hand, is involved in maintaining balance.
Why do you run
The impedance test studies the functionality of the sound transmission system within the auditory system. This investigation contributes to the diagnosis of ear diseases and allows the patient to measure the hypoacusis ( decrease in hearing ), especially from a qualitative point of view.
More in detail, the exam allows to evaluate the following parameters with a single device, the impedance meter:
- Impedance of the tympanic-ossicular system (tympanogram) : consists of the study of the elasticity of the tympanic membrane and the degree of movement of the internal ossicles (hammer, anvil and stirrup). In other words, this parameter can be expressed as the resistance opposed by the middle ear to the passage of the sound wave.
- Stapedial reflex (reflexogram) : provides information on stapedius reflexes (very small stirrup muscle, located in the middle ear) and on the integrity of the central acoustic pathway (acoustic nerve and nuclei). In subjects without pathological situations, the stapedius muscle contracts in response to loud sounds. This phenomenon is bilateral, even when only one ear is stimulated. The contraction of the stapedius muscle corresponds to an increase in the rigidity of the eardrum, which thus reduces the sound conduction towards the inner ear. The absence or presence of the stapedial reflex is an index of fundamental importance for the diagnosis of various pathologies, such as otitis media, otosclerosis and sensorineural hearing loss.
To remember
Physiologically, the stapedial reflex has the function of protecting the auditory components from too intense sound stimulations and also plays a role in the ability to discriminate sounds.
When do you run?
Impedance measurement is indicated to assess the subject's hearing ability by studying the behavior of the external and middle ear when they are hit by an artificially produced sound. The procedure is particularly useful when it is necessary to understand from which type of hearing loss the patient is affected and helps to diagnose some diseases, such as otosclerosis or tubal dysfunction.
In otorhinolaryngology, the impedance test is one of the so-called " objective tests ", since it allows an evaluation of the auditory system without the need to have the collaboration or the answers of the patient (therefore it can be performed also on small children, sick in coma and etc).
Once the therapy has been assigned, the impedance measurement allows the doctor to monitor the evolution of the disorder.
Associated examinations
Usually, to understand the nature of an audiological problem, impedance measurement is often complementary to an audiometric examination. Depending on the results obtained, these investigations may be associated with other investigations aimed at deepening the clinical picture.
Altered Values - Causes
The impedance test is able to provide information on the involvement of the middle ear structures, in the pathologies that interest it.
In particular, this investigation is extremely useful in the diagnosis of:
- Otitis media (in particular, in the suspicion of serous effusions, accumulation of catarrh in the ear etc.);
- External ear plugs;
- Otosalpingites (inflammations of the Eustachian tube);
- Tubal insufficiency (alteration of the activity of the Eustachian tube);
- Perforation of the eardrum;
- Otosclerosis or ossicular discontinuity (blockage of the motility of the ossicular chain);
- tympanosclerosis;
- Hypermobility of the tympanic membrane;
- Neurosensory hypoacusis (discrimination between cochlear pathology and / or acoustic nerve);
- Acoustic neuroma or other pathologies of the central acoustic pathway (ie the auditory nerve and nuclei)
- Facial nerve disorders;
- Some disorders of the brain stem.
How to do it
Impedance measurement is an instrumental exam performed by an otolaryngologist (specialist in throat, nose and ear diseases) on an outpatient basis. The procedure is painless and non-invasive.
Basic mechanism
The impedance test includes the positioning inside the acoustic meatus of a probe, equipped with a soft plastic tip, until hermetic closure is obtained.
Subsequently, the instrument generates a pure tone (usually 226 Hz) and measures the sound reflection from the tympanic membrane, while the operator varies the air pressure in the external auditory canal. The impedance meter emits a sound pressure of variable entity, positive and negative, from a maximum level of +200 decapascals (daPa) to a minimum of -400 daPa, capable of moving the eardrum and the chain of ossicles attached to it ; at the same time, a microphone picks up and transduces sound in direct current.
The instrument thus registers the resistance opposite to the passage of air in the middle ear, at different pressure values. With severe sounds, impedance is a function of rigidity and its inverse, that is compliance or, in technical terms, compliance (note: compliance indicates how energy is transmitted in the middle ear, depending on pressure changes) .
The maximum peak of compliance occurs when the air pressures of the ear canal and middle ear are equal, thus maximizing acoustic transmission through the middle ear. The peak of compliance, therefore, indicates the pressure of the middle ear and implies the effectiveness of the function of the Eustachian tube.
If the tympanogram is normal, the stapedial reflexes are examined: the device emits intense sounds, which stimulate the contraction of the stapedius muscle, recording this movement with the impedometer.
Who does it run from?
The examination is performed by an otolaryngologist specialist, with the support of qualified operators who, during their professional training, have acquired specific experience on the physiopathology of the ear.
How long does it last?
The duration of the exam is variable: normally, for the execution of the impedance measurement it takes approximately 5 minutes.
What does the report contain?
In the report, the diagnostic conclusion is given, with any iconographic documentation attached (graphs related to the tympanogram and stapedial reflex).
Preparation
Impedance measurement should always be performed after a specialist examination, during which the doctor performs an otoscopic examination (otoscopy). In fact, a fundamental prerequisite for the correct execution of the examination is that the external auditory canal is patent and free of obstructions in the ear canal, such as, for example, a wax plug.
Contraindications
Impedance measurement is painless, easily repeatable, very simple in its execution and non-invasive.
However, some conditions can influence the test result, making the evaluation not very useful.
Contraindications to impedance measurement include:
- Perforation of the tympanic membrane;
- Acute otitis media and other acute inflammatory processes (they can determine a marked weakening of the tympanic membrane);
- Excessive presence of earwax.
In cases where it is not possible to intervene to remove the obstacle to the transmission system, the otolaryngologist will consider postponing the evaluation to another time.
Impedance measurement is not recommended for children under the age of 7 months, as the cartilage of their ear canal is weak and tympanometric values can be misleading.
Interpretation of results
As for the tympanogram, the impedance measurement of the ear can provide the following results:
- Type A: this type of path indicates that the auditory system works perfectly, ie the mobility of the eardrum-ossicular system is normal, as is the passage of sound in the middle ear.
- Type As : the mobility of the transmission system is reduced; this finding is typical of diseases, such as tympanosclerosis.
- Type Ad: the mobility of the transmission system is excessive; this result is usually indicated by a flaccidity of the tympanic membrane (which occurs, for example, in the case of partial atrophy of the eardrum or detachment of the ossicular chain).
- Type B : the motility of the transmission system is zero. This type of graph is typically found in the presence of catarrhal middle ear infections.
- Type C : the trace indicates that the pressure inside the tympanic case is negative. This signals the presence of tubal dysfunction and retraction of the eardrum due, for example, to a serous effusion.
As far as the search for stapedial reflexes is concerned, on the other hand, the impedance test allows us to distinguish the different types of hearing loss. For example, patients with a transmissive lesion usually have an absent stapedial reflex.
In conclusion, based on the results of the examination, the doctor will be able to detect any disturbances affecting the auditory perception and to understand the nature of the hearing problem.
In combination with audiometry and otoneurological tests, the impedenzometric examination of the ear is of fundamental importance for the diagnosis of diseases such as otitis media, otosalpingites, tympanic perforations, otosclerosis, sensorineural hearing loss and other pathologies of the facial nerve and central acoustic pathway.
